1. Field
The presently disclosed embodiment relates to an UV-LED curable clear ink composition for ink jet printing. In particular, the presently disclosed embodiment relates to an UV-LED curable clear ink composition for ink jet printing which is excellent in curability (UV-LED curability) by ultraviolet light (UV) using a light emitting diode (LED) as a light source, ejection stability, and adhesion to a substrate and scratch resistance of an obtained cured coating film, and assures that yellowing of the cured coating film is hardly generated.
2. Brief Description of Related Developments
In an ink jet recording apparatus, a method (an ink jet method) of ejecting ink from a nozzle and adhering the ink on a substrate. In this method, the nozzle and the substrate are in a non-contacted state. Therefore according to this method, satisfactory printing can be performed on surfaces of a vinyl chloride resin sheet, a tarpaulin sheet of a composite material, substrates having irregular shapes such as a curve, a convex and a concave, and the like. As a result, an opportunity of using an ink jet recording apparatus in the fields of industrial application such as a large size outdoor advertisement having a large image area is increasing. Known examples of ink composition for ink jet printing to be used in such an ink jet recording apparatus include an aqueous ink composition for ink jet printing, an oil ink composition for ink j et printing and an ultraviolet light curable ink composition for ink jet printing to be curable by ultraviolet light or the like. Among these, an ultraviolet light curable ink composition for ink jet printing is excellent in drying characteristics and curability of a coating film even in the case of a non-absorbable substrate.
However, in an image formed by an ink jet method, a surface of the image becomes uneven, and uneven brightness which results from a difference in a reflectance between the substrate portion not printed with ink and the ink-printed image portion is likely to arise. Therefore, quality of the image formed by an ink jet method is deteriorated due to such uneven surface and uneven brightness. In addition, in the image formed by an ink jet method, its smoothness is apt to be deteriorated by the formed uneven surface, and it is difficult to increase brightness.
Therefore, in order to eliminate such uneven brightness and low brightness, there was proposed a technology of printing, after printing on a substrate, an ultraviolet light-curable clear ink composition for ink jet printing by an ultraviolet light-curable ink jet printing method. According to this technology, a surface of an obtained image is homogenized, smoothness is increased, uneven brightness is eliminated, and a brightness is increased.
Meanwhile, high energy light sources such as a metal halide lamp have been used as a light source for curing an ultraviolet light-curable ink composition for ink jet printing. When an image is cured by using such a lamp, there are problems with generation of ozone, increase in size of a light irradiation device, a short life of a lamp, and the like. Therefore, recently a low energy light emitting diode has been used.
Further, Japanese Patent Document No. 2011-057744 discloses an ultraviolet light-curable clear ink composition for ink jet printing comprising a polymerization inhibitor such as a hindered amine compound or a nitrosamine compound, and a polymerization initiator such as acylphosphine oxide compound as a conventional ultraviolet light-curable clear ink composition for ink jet printing. Furthermore, Japanese Patent Document No. 2013-503931 discloses an ultraviolet light-curable clear ink composition for ink jet printing comprising at least 30% by mass of a mono-functional acrylate monomer and at least 3% by mass of a photo initiator based on a total mass of the ink, wherein the photo initiator comprises an α-hydroxyketone photo-polymerization initiator and 0 to 6% by mass of an acylphosphine photo-polymerization initiator based on a total mass of the ink.